1. Field of the Invention
The present invention relates to a refrigerating system which supplies a gaseous refrigerant of high pressure to an evaporator to defrost the evaporator and also supplies a liquid refrigerant to a low pressure side of the interior of a compressor through a liquid injection circuit to effect cooling of the compressor.
2. Description of the Prior Art
Heretofore, in a showcase for refrigeration and cold storage mounted as a food refrigerating and cold storage equipment in a supermarket or the like, there has been adopted a method of using a high-pressure gas refrigerant discharged from a compressor, for defrosting an evaporator as a constituent of the refrigerator. There also has been adopted a so-called liquid injection method in which a liquid refrigerant is fed to the interior of a compressor and is allowed to evaporate therein to cool the compressor for the purpose of preventing the increase of the temperature of gas discharged from the compressor.
FIGS. 3 to 5 are refrigerant circuit diagrams in conventional refrigerating systems of this type. FIG. 3 illustrates a refrigerating system of the type in which a refrigerant is condensed by cooling with air, and a gaseous refrigerant of high pressure discharged from a compressor during defrosting is allowed to flow directly through an evaporator. FIG. 4 illustrates a refrigerating system of the type in which a refrigerant is condensed by cooling with water, and like FIG. 3, a gaseous refrigerant of high pressure discharged from a compressor is allowed to flow directly through an evaporator during defrosting. FIG. 5 illustrates a refrigerating system of the type in which a refrigerant is condensed by cooling with air, and the refrigerant in a gas-liquid mixed state leaving a condenser during defrosting is allowed to flow into an evaporator. In these figures, the portions indicated by the same reference numerals represent the same portions.
Referring first to FIG. 3, a discharge-side pipe 2 is connected to a refrigerant discharge side 1D of a compressor constituted by a scroll compressor or a semi-sealed type compressor, and it is also connected at an opposite end thereof to a refrigerant inlet side 3A of an air-cooled condenser 3. To a refrigerant outlet side 3B of the condenser 3 is connected an outlet-side pipe 4, which is connected at an opposite end thereof to a refrigerant inlet side 5A of a receiver tank 5. To a refrigerant outlet side 5B of the receiver tank 5 is connected an outlet-side pipe 6, to which are connected in series a drier 7, a sight glass 8, a valve 9, and solenoid valves 10, 11. The solenoid valve 11 is connected to an evaporator 13 through an expansion valve 12.
The evaporator 13 is mounted in an inner cold air passage of a showcase for refrigeration and cold a storage (not shown), and an outlet side of the evaporator 13 is connected to an accumulator 16 through a solenoid valve 14 and further through a low pressure-side pipe 15. A solenoid valve 18 is disposed in a by-pass pipe 17 which by-passes the solenoid valve 11 and the expansion valve 12, and a pipe 19 branching from between the solenoid valve 11 and the expansion valve 12 is connected to an evaporator 22 through a solenoid valve 20 and an expansion valve 21. The evaporator 22 is mounted in an outer cold air passage of the showcase for refrigeration and cold storage, and an outlet side thereof is connected to low pressure-side pipe 15. A pipe 24 branching from between the evaporator 13 and the solenoid valve 14 is connected to an inlet side of the solenoid valve 20 through a check valve 25. Further, a suction-side pipe 26 connected to an outlet side of the accumulator 16 is connected in an opposite end thereof to a suction side 1S of the compressor 1.
A liquid injection circuit 27 branches from the outlet-side pipe 6 of the receiver tank 5 and is connected to a liquid injection inlet 1R on a low pressure side in the compressor 1 through a capillary tube 28 and a solenoid valve 29. A defrosting pipe 30 branching from the discharge-side pipe 2 of the compressor 1 is connected to an outlet side of the solenoid valve 10 through a solenoid valve 31. Further, a pipe 32 branched from the discharge-side pipe 2 is connected to the low pressure-side pipe 15 through a solenoid valve 33 and a low-pressure regulating valve 34.
The operation of the refrigerating system shown in FIG. 3 will now be described. During normal cooling operation using the evaporator 13, the solenoid valves 10, 11, 14 and 29 are open, while the other solenoid valves are closed. The gaseous refrigerant of high temperature and high pressure discharged from the compressor 1 radiates heat and condenses in the condenser 3, then the refrigerant, which is now in a gas-liquid mixed state, flows into the receiver tank 5, in which the refrigerant is separated into gas and liquid. The liquid refrigerant, present in the lower portion, flows out from the outlet side 5A, passes through the outlet-side pipe 6, further passes through the solenoid valves 10 and 11, then is throttled by the expansion valve 12 and thereafter enters the evaporator 13, as indicated by solid-line arrows in the figure. The refrigerant evaporates in the evaporator 13, then passes through the solenoid valve 14, further through the low pressure-side pipe 15, and enters the accumulator 16, in which unevaporated liquid refrigerant is separated. Only the gaseous refrigerant is introduced into the compressor 1.
After such cooling operation has been done for a predetermined period of time (e.g. 3 hours), there is performed a defrosting operation for the evaporator 13. However, prior to starting the defrosting operation, the solenoid valve 20 is opened to a greater extent than the foregoing state thereof only for a predetermined short period (e.g. 30 seconds), thereby allowing the refrigerant which has been throttled by the expansion valve 21 to allow also into the evaporator 22 for evaporation therein, as indicated by broken-line arrows in the figure. Thus, the interior of the showcase is cooled by both evaporators 13 and 22 which are for the inner and outer cold air passages, respectively. After completion of this cooling operation, the solenoid valves 31, 18, 20, 29 and 33 are opened, while the other solenoid valves are closed. As a result, the gaseous refrigerant of high temperature and high pressure discharged from the compressor 1 passes through the defrosting pipe 30, further through the solenoid valves 31 and 18, while by-passing the expansion valve 12 through the by-pass pipe 17, and enters the evaporator 13, as indicated by broken-line arrows in the figure. Consequently, the evaporator 13 is heated and defrosted. At the same time, the refrigerant condensed in the interior passes through the pipe 24, further through the check valve 25 and the solenoid valve 20, then is throttled in the expansion valve 21, thereafter flows into the evaporator 22 and is evaporated therein. Thus, even during defrosting of the evaporator 13, the interior of the showcase can be cooled by the evaporator 22. The refrigerant evaporated in the evaporator 22 returns to the accumulator 16 in the same manner as described above. During defrosting, moreover, the gaseous refrigerant of high temperature and high pressure discharged from the compressor 1 passes through the solenoid valve 33 and the low-pressure regulating valve 34 and flows into the suction-side pipe 15 to prevent the low pressure-side pressure of the compressor 1 from dropping too much.
A defrosting end temperature of the evaporator 13 is sensed by a sensor (not shown), and when the defrosting of the evaporator 13 is completed, only the solenoid valves 20 and 29 are opened for a predetermined period (e.g. 3 minutes), while the other solenoid valves are closed, whereby there is performed an operation for recovering the refrigerant present in each of both evaporators 13 and 22.
Since the solenoid valve 29 is kept open over each of the above operation periods, the liquid refrigerant staying in the receiver tank flows through the liquid injection circuit 27, then is throttled by the capillary tube 28 and enters the compressor 1, where it is evaporated and cools the compressor 1 to cool the oil, compressed refrigerant, motor core and the other parts in the compressor 1.
In the refrigerating system shown in FIG. 4, the foregoing condenser 3 is not present, and a discharge-side pipe 2 connected to a discharge side 1D of the compressor 1 is connected in an opposite end thereof to a refrigerant inlet side 5A of a receiver tank 5 through a drier 36. On the other hand, a water-cooling pipe 37 through which cooling water flows is drawn into the receiver tank 5. The refrigerant present in the receiver tank 5 is cooled and condensed by the water-cooling pipe 37. The flow of water into the pipe 37 is controlled by the pressure discharged from the compressor 1 in such a manner that water flows upon increase of the pressure and stops upon decrease thereof. Other constructional and operational points are the same as in FIG. 3.
Next, in the refrigerating system shown in FIG. 5, an outlet-side pipe 4 of a condenser 3 is connected to a refrigerant inlet side 5A of a receiver tank 5, and defrosting pipe 30 branches from the outlet-side pipe 4 in a position between the condenser 3 and a check valve 39. An auxiliary accumulator 40 is disposed in a low pressure-side pipe 15. In this case, a gas-liquid mixed refrigerant after the removal of rough heat and condensed in the condenser 3 flows into the defrosting pipe 30 and is used for defrosting an evaporator 13. Other constructional and operational points are the same as in FIG. 3.
In each of the above refrigerating systems, a predetermined amount of a refrigerant, e.g. R-22 or R-50, is sealed into the refrigerant circuit, but since the defrosting pipe 30 by-passes the receiver tank 5, the amount of the refrigerant flowing into the receiver tank 5 during defrosting of the evaporator 13 becomes smaller. Particularly, in the refrigerating system of FIG. 5, most of the gas-liquid mixed refrigerant leaving the condenser 3 flows through the defrosting pipe 30, resulting in that the amount of liquid refrigerant staying in the receiver tank 5 during defrosting decreases to an amount of 1 to 2 liters.
However, for cooling the compressor 1 it is necessary to flow a liquid refrigerant through the liquid injection circuit 27 at a rate of 600 cc or so per minute. During defrosting of the evaporator 13, therefore, the liquid refrigerant in the receiver tank 5 will be exhausted in an early stage, with the result that the liquid refrigerant to be fed to the liquid injection circuit 27 becomes short and the temperature of the compressor 1 rises. Since the rise in temperature of the compressor 1 causes damage to the compressor 1, a protective device (not shown) operates to stop the operation of the compressor 1.
Actually, experiments were conducted using a refrigerant sealed in the refrigerating systems in an amount so small as to evolve flash gas in the sight glass 8 portion. As a result, in the refrigerating system of FIG. 5, the head temperature of the compressor 1 during defrosting exceeded +120.degree. C. and the protective device operated to stop the operation of the compressor. Once the operation of the compressor 1 stops, there arises the problem that the defrosting of the evaporator 1 is also discontinued.
Also in the refrigerating system of FIG. 3 or FIG. 4, since the gaseous refrigerant of high temperature and high pressure discharged from the compressor 1 by-passes the receiver tank 5 and flows through the defrosting pipe 30, the amount of the liquid refrigerant flowing through the liquid injection circuit 27 became insufficient, and although the operation of the compressor 1 did not stop, the head temperature of the compressor also exceeded +120.degree. C. In this state, the operation of the compressor became extremely unstable.
For defrosting an evaporator using a gaseous refrigerant of high pressure, there also has been proposed a method of using a gaseous refrigerant after gas-liquid separation in a receiver tank, as disclosed in Japanese Patent Publication No. 20022/74 for example.